Oil country tubular goods (such as tubing, through which crude oil or other fluid flow, and casing which surrounds tubing) used for excavating oil wells for exploitation of crude oil and natural gas typically have a length of ten some meters and are connected using a threaded joint until a length reaching an oil well is achieved. In the past, the depth of oil wells was 2,000-3,000 meters. However, recently, in deep oil wells such as for undersea oil fields, the depth of oil wells may reach 8,000-10,000 meters or more.
In its environment of use, a threaded joint used for connecting oil country tubular goods is acted upon by loads such tensile forces in the axial direction caused by the weight of oil country tubular goods and the threaded joints themselves, compound pressure due to internal and external surface pressure, and geothermal heat. Therefore, a threaded joint for steel pipes needs to be able to maintain the gastightness of oil country tubular goods without being damaged even in such a severe environment.
A typical threaded joint used for connecting oil country tubular goods has a pin-box structure such as schematically shown in FIG. 1. A pin 1 is a joint component having a male or external thread 3a which is typically formed on the end of an oil country tubular good A. A box 2 is a joint component having a female or internal thread 3b which is typically formed on the inner surface of a threaded joint member B (a coupling). Unthreaded metal contact portions are formed close to the distal end of the male thread 3a of the pin 1 and close to the proximal end of the female thread 3b of the box 2. Gastightness of the oil country tubular good A is guaranteed by inserting one end of the oil country tubular good A into the threaded joint member B and tightening the male threads 3a and the female threads 3b so that the unthreaded metal contact portions of the pin and the box contact each other.
During the process of lowering tubing or casing into an oil well, due to various problems, a threaded joint which has previously been connected is sometimes lifted out of the oil well, retightened, and then lowered into the well. API (American Petroleum Institute) requires galling resistance and gastightness such that even if makeup (tightening) and breakout (loosening) are carried out ten times for a joint for tubing and three times for a joint for casing, there is no occurrence of unrepairable seizing referred to as galling and the gastightness of oil country tubular goods is maintained.
In order to increase gastightness and galling resistance at the time of makeup, in the past, a viscous liquid lubricant containing heavy metal powders (referred to as compound grease) was applied to the contact surfaces (the threaded portions and the unthreaded metal contact portions) of a threaded joint. Such a compound grease is prescribed by API BUL 5A2. Compound grease also exhibits corrosion resistance (rust-preventing properties) in that it prevents the formation of rust on the contact surfaces to which it is applied.
With the object of increasing the retention of compound grease (the adhesion of grease to the contact surfaces of a threaded joint) and improving the lubricating properties of a threaded joint, it has been proposed to perform various types of surface treatment such as nitriding, plating (such as zinc-based plating or dispersion plating), or phosphating treatment to form one or more surface treatment layers on the contact surfaces of a threaded joint.
However, the use of compound grease has the problem that there is a concern of adverse effects on the environment and humans. Compound grease contains a large amount of heavy metal powders such as zinc, lead, and copper. Therefore, at the time of makeup of a threaded joint, the applied grease is washed off or overflows to the outer surface, and there is the possibility of harmful heavy metals such as lead and the like having an adverse effect on the environment (particularly sea life). In addition, the process of applying compound grease worsens the work environment, so there is a concern of toxic effects on the human body.
In recent years, as a result of the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) relating to preventing maritime pollution in the Northeast Atlantic, strict restrictions with respect to the environment are advancing on a global scale, and the use of compound grease is already being restricted in some regions.
Accordingly, in order to avoid adverse effects on the environment and humans in the excavation of gas wells and oil wells, a demand for a threaded joint which can exhibit excellent galling resistance without using compound grease has developed.
Another problem of compound grease is that it contains a large amount of a solid lubricant typified by graphite, and it forms a coating which is not transparent. A pin having a threaded portion on the outer surface of a tubular body more easily undergoes damage during transport or at the time of makeup than does a box having a threaded portion on the inner surface of a tubular body. Therefore, a pin is often subjected to visual inspection for damage to the threaded portion thereof prior to makeup operations in order to avoid the occurrence of sudden galling caused by damage to the threaded portion of the pin which is formed on the outer surface of a pipe. When a compound grease has been applied, it was necessary at the time of inspection to clean the pin by washing the applied compound grease off and then to reapply compound grease after inspection. As described above, such operation is harmful to the environment and is time-consuming. If the coating were transparent, the threaded portion could be visually inspected for damage without removing the applied coating, and the labor required for inspection could be greatly decreased.
After an oil country tubular good is manufactured, it is sometimes stored for a number of months or longer until it is actually used. Therefore, “storage grease” is applied to the contact surfaces of a threaded joint. Like compound grease, storage grease is not transparent, so each time inspection is carried out, it is necessary to wash it off. Therefore, like compound grease, storage grease has an environmental problem.
In below-described Patent Documents 1-3, one of the present applicants proposed the following threaded joints which can be used to connect oil country tubular goods without applying compound grease or storage grease.
Patent Document 1 (WO 2006/104251): A threaded joint in which the contact surfaces of at least one of a pin and a box are coated with a coating having a two-layer structure (a two-layer coating) which is constituted by a lower viscous liquid or semisolid lubricating coating and an upper dry solid coating. The dry solid coating can be formed from a thermosetting resin such as an acrylic resin or an ultraviolet curable resin. Since the viscous liquid or semisolid lubricating coating is tacky, foreign matter easily adheres to it, but by forming a dry solid coating atop it, its tackiness is eliminated. The dry solid coating is destroyed at the time of makeup of a threaded joint, and this upper coating does not impair the lubricating properties of the lubricating coating beneath it.
Patent Document 2 (WO 2007/042231): A threaded joint having a thin, non-tacky lubricating coating formed on a threaded portion (e.g., of a pin or a box). The lubricating coating contains solid lubricant particles dispersed in a solid matrix exhibiting plastic or viscoplastic rheological properties (flow properties). The solid matrix preferably has a melting point in the range of 80-320° C. This lubricating coating is formed by spray coating in a molten state (hot melt spraying), flame spraying using a powder, or spray coating of an aqueous emulsion. A composition used for hot melt spraying comprises polyethylene as a thermoplastic polymer, wax (such as carnauba wax) and a metal soap (such as zinc stearate) as a lubricant component, and calcium sulfonate as a corrosion inhibitor.
Patent Document 3 (WO 2006/075774): A threaded joint in which a contact surface of at least one of a pin and a box is coated with a two-layer coating constituted by a lower solid lubricating coating comprising a lubricant powder and a binder, and an upper solid anticorrosive coating which does not contain solid particles.
In addition, Patent Document 4 (JP 2002-080511 A1) discloses a photocurable composition which comprises (A) a photocurable (meth)acrylate resin, (B) a carboxyl group-containing monofunctional (meth)acrylate monomer, (C) a (meth)acrylate phosphate compound, (D) a difunctional (meth)acrylate monomer, (E) a trifunctional or higher multifunctional (meth)acrylate monomer, (F) a photopolymerization initiator, and optionally an anticorrosive pigment. In an example of Patent Document 4, a composition is illustrated in which a condensed phosphate aluminum salt is used as an anticorrosive pigment. According to Patent Document 4, by using the photocurable composition disclosed therein, a coating having excellent properties in terms of adhesion to a steel pipe, rust prevention, and surface smoothness can be formed.
Patent Document 1: WO 2006/104251
Patent Document 2: WO 2007/042231
Patent Document 3: WO 2006/075774
Patent Document 4: JP 2002-080511 A1